This invention relates to an illumination system with a light source such as a high pressure Hg lamp, for example, wherein light is emitted in a range having a certain size or area. More particularly, the invention concerns an illumination system suitably usable in a projection exposure apparatus for the manufacture of semiconductor devices or liquid crystal panels, for example, in which illumination of a rectangular region is required for a reticle to be used in an exposure sequence.
Projection exposure apparatuses use an illumination system for uniformly illuminating a surface to be illuminated, by use of a light source such as a high pressure Hg lamp wherein light is emitted from a predetermined range (a range having a certain size or area) with a predetermined distribution, as disclosed in Japanese Laid-Open Patent Application, Laid-Open No. 333795/1994. Referring to FIG. 29, an example using such an illumination system will be described. Denoted in the drawing at 1 is a light source, and denoted at 2 is light collecting means for collecting light emitted from the light source 1 with a predetermined angular distribution. In this example, the collecting mirror uses an elliptical mirror. The light source 1 is disposed adjacent to a first focal point of the elliptical mirror, and the light emitted from the light source 1 is collected adjacent to a second focal point 00.
In the example of FIG. 29, an optical integrator is used to provide Koehler illumination, by which uniform illumination is accomplished. Namely, the light collected in the vicinity of the second focal point 00 of the elliptical mirror (collecting means 2) is transformed by a collimator lens 3 into parallel light which then enters an optical integrator 4 in a parallel beam. The optical integrator 4 serves to produce a number of light convergence points about a light exit end thereof. In the example of FIG. 29, these light convergence points function as secondary light sources which with and, through an optical system 5, an illumination region 6 is uniformly illuminated.
In order that the illumination is made in accordance with the shape of the illumination region, because of Koehler illumination, a fly""s eye lens (as the optical integrator 4) may be designed so that the emission angle from the integrator corresponds to the illumination position. For example, if the illumination region has a square shape, a fly""s eye lens comprising an array of element lenses having a square sectional shape, as shown in FIG. 30, may be used. If the illumination region has a rectangular shape, a rectangular fly""s eye lens such as shown in FIG. 31A, comprising element lenses of rectangular shape having the same sectional aspect ratio as that of the illumination region, may be used.
This is because, when parallel light is projected on a rectangular fly""s eye lens, as schematically shown in FIGS. 31B and 31C, the incidence height of light rays upon the light entrance face is higher in the lengthwise section in FIG. 31B than in the widthwise section in FIG. 31C, such that the largest value of the angle defined between the light ray emitted from the exit face and the optical axis becomes larger in the lengthwise direction. Namely, xc3x8b greater than xc3x8c. By performing Koehler illumination while using this emitted light, a rectangular region can be illuminated efficiently.
As regards constituent elements of optical systems where the system is used in a projection exposure apparatus, denoted in FIG. 29 at 7 is a projection lens system. Denoted at 8 is a stop of the projection lens system 7, and denoted at 9 is a wafer substrate.
However, when a fly""s eye lens is used as an optical integrator, as shown in FIG. 32, there is a certain limit to the incidence angle of light that can be emitted from each element lens of the fly""s eye lens. More specifically, while a light ray being incident with a certain incidence angle xe2x80x9caxe2x80x9d can be emitted from the light exit face of the fly""s eye lens, another light ray being incident with an incidence angle xe2x80x9cbxe2x80x9d larger than a certain angle cannot be emitted from the fly""s eye lens because it is eclipsed by the side face of the element lens.
If light rays collected by the light collecting means 2 are completely converged at a convergence point, as described above, a completely parallel light flux can be projected on the fly""s eye lens by using a collimator lens and, therefore, all the light rays emitted from the light source can be projected out of the fly""s eye lens. Thus, the secondary light sources can be produced very efficiently. However, in a light source such as a high pressure Hg lamp, light rays are emitted from a predetermined range having a certain size or area, and all the light rays emitted from the light source cannot be completely collected at the second focal point only by use of a single elliptical mirror. In other words, the light rays reflected by the elliptical mirror necessarily have a particular angular distribution and a particular positional distribution, on a plane orthogonal to the optical axis and adjacent to the second focal point of the elliptical mirror.
As described, when a light source such as a high pressure Hg lamp is used, the light rays emitted from the light source cannot be collected at a single point and, therefore, all the light rays cannot be transformed into a parallel light flux even by use of a collimator lens. As a result of it, upon the light entrance surface of a fly""s eye lens, the light rays necessarily have a particular angular distribution. Therefore, the relationship between the incidence angle of light upon the fly""s eye lens and the capability of emission thereof from the lens must be considered. If there exist some rays that cannot be emitted from the fly""s eye lens, the illuminance may be lowered by the lens.
Next, an illumination system is discussed, which is arranged so that light rays (light source image) distributed at the light convergence point are imaged upon a fly""s eye lens to assure uniform illumination on a surface to be illuminated. This, however, does not narrow the scope of the present invention. The way of making light rays, being distributed about the second focal point, to enter a fly""s eye lens can be chosen freely. It can be done within the scope of the present invention to make light enter a fly""s eye lens while using a collimator lens such as shown in FIG. 29.
As described above, when a fly""s eye lens comprising plural element lenses is used in relation to an illumination region of rectangular shape, the largest value for the incidence angle of light that can be emitted from the lens differs between that in the lengthwise direction and that in the widthwise direction of the rectangular shape. More specifically, the largest incidence angle xcex8b of light that can be emitted, with respect to the lengthwise direction of FIG. 31B, is larger than the largest incidence angle xcex8c of the light that can be emitted, with respect to the widthwise direction of FIG. 31C. Nevertheless, as regards the light fluxes being collected by conventional light collecting means toward the convergence point, because the elliptical mirror which is used conventionally as the light collecting means has a shape being revolutionally symmetrical with respect to the optical axis, as shown in FIG. 4, the light rays emitted from the first focal point of the elliptical mirror define a cone about the optical axis, with its apex placed at the convergence point 00. Although those to be taken into account are light rays which are not collected at the second focal point, since those light rays emitted from positions away from the first focal point go beside the cone and pass near the second focal point, only the cone defined by the light rays emitted from the first focal point is discussed here as a representative example. Also, in the following description, while only the light rays emitted from the first focal point will be discussed, those light rays emitted from positions away from the first focal point are distributed near a cone defined by the light rays from the first focal point and, therefore, a similar discussion applies.
In summary, when a conventional light collecting mirror of revolutionally symmetrical shape is used, the angular distribution of light at the convergence point is even with respect to both of the lengthwise direction and the widthwise direction. Thus, when the thus collected light is used in combination with a fly""s eye lens for efficient illumination of a rectangular region, all the light rays must be made incident on the fly""s eye lens at a small angle, being smaller than the largest incidence angle xcex8c in relation to the widthwise direction for emission from the lens, such that all the rays can be emitted from the fly""s eye lens.
However, in an optical system, there is a limitation due to an Helmholtz-Lagrange relationship. That is as, also called Helmholtz-Lagrange invariable, in an optical system, the product of the angle between two intersecting light rays and the distance of the intersection from the optical axis is reserved. As described above, since the light rays emitted from positions off the first focal point are not collected at the convergence point, there is a Helmholtz-Lagrange invariable being not zero.
This means that, when the incidence angle upon the light entrance surface of the fly""s eye lens (which is at the image point) is made smaller, it results in that the image height becomes larger. Namely, the area of the light entrance surface of the fly""s eye lens becomes larger. The area of the light entrance surface of a fly""s eye lens corresponds to the size of an effective light source in an illumination system. Thus, in an illumination system having a definite size of an effective light source, the area of the light entrance surface of the fly""s eye lens is determined fixedly and, therefore, it is not possible to make the incidence angle smaller than a certain angle. As a result, some light rays are eclipsed at the side face of each element lens and they cannot be emitted from the fly""s eye lens. The efficiency of the light is, therefore, not good.
It is an object of the present invention to provide an illumination system with improved light collecting means by which even a rectangular illumination region can be illuminated with a good efficiency.
In one preferred form of the present invention, an illumination system comprises light collecting means for collecting light divergently emitted from a light source having a certain size, and a fly""s eye lens as an optical integrator for defining an illumination region of rectangular shape, having different lengths with respect to two orthogonal directions, the fly""s eye lens including plural element lenses arrayed along the two orthogonal directions and having a rectangular sectional shape with different lengths with respect to the two orthogonal directions, wherein the light collecting means is arranged so that a largest light convergence angle defined between an optical axis and the light collected by the light collecting means differs with respect to two orthogonal directions.
In one preferred form, the light collecting means may include a main reflection mirror for reflecting and directing the light from the light source to the optical integrator, and an auxiliary reflection mirror for directing light, not directly impinging on the main reflection mirror, and directing the same to the main reflection mirror. The main reflection mirror may comprise at least one elliptical mirror having a focal point placed at about a center of the light source. The auxiliary reflection mirror may comprise at least one of a spherical mirror having its center placed at about the first or second focal point of the main reflection mirror, a hyperboloids mirror having one focal point placed at about the first focal point of the main reflection mirror, an elliptical mirror having one focal point placed at about the first focal point of the main reflection mirror, a parabolic mirror having a focal point placed at about the first focal point of the main reflection mirror, and a plane mirror. The optical components following the light collecting means may preferably be arranged so that the largest convergence angle of light collected by the light collecting means becomes symmetrical with respect to the optical axis.
In one preferred form, the light collecting means may include a main reflection mirror for reflecting and directing the light from the light source to the optical integrator, and an auxiliary reflection mirror for directing light, not directly impinging on the main reflection mirror, and directing the same to the main reflection mirror. The main reflection mirror may comprise at least one elliptical mirror having a focal point placed at about a center of the light source, and at least one hyperboloids mirror having two focal points placed substantially at the same positions as the two focal points of the elliptical mirror, respectively. The auxiliary reflection mirror may comprise at least one of a spherical mirror having its center placed at about the first or second focal point of the main reflection mirror, a hyperboloids mirror having one focal point placed at about the first focal point of the main reflection mirror, an elliptical mirror having one focal point placed at about the first focal point of the main reflection mirror, a parabolic mirror having a focal point placed at about the first focal point of the main reflection mirror, and a plane mirror. The optical components following the light collecting means may preferably be arranged so that the largest convergence angle of light collected by the light collecting means becomes symmetrical with respect to the optical axis.
In an illumination system according to any one of these preferred embodiments, the main reflection mirror and the auxiliary reflection mirror may preferably have a shape symmetrical with respect to a plane containing the optical axis, of the optical system following the light collecting means, and being parallel to the widthwise direction of the two orthogonal directions.
In one preferred form of the present invention, the main reflection mirror may comprise an elliptical mirror having one focal point placed adjacent to the light source, and having such a shape that the mirror is placed only at one side of a plane which contains a straight line connecting two focal points of the elliptical mirror and which is parallel to the lengthwise direction of the two orthogonal directions.
In one preferred form of the present invention, the auxiliary reflection mirror may comprise a spherical surface mirror having its center placed adjacent to the center of the light source. The elliptical mirror and the spherical mirror may be disposed with a boundary adjacent to a plane which contains a straight line connecting the two focal points of the elliptical mirror and which is parallel to the lengthwise direction of the two orthogonal directions. Alternatively, the auxiliary mirror comprises at least two parabolic mirrors having a focal point adjacent to the center of the light source, and the elliptical mirror and the two parabolic mirrors may be disposed with a boundary adjacent to a plane which contains a straight line connecting the two focal points of the elliptical mirror and which is parallel to the lengthwise direction of the two orthogonal directions.
In one preferred form of the present invention, the auxiliary reflection mirror may comprise a plane mirror and a parabolic surface mirror having its focal point placed adjacent to the center of the light source. The elliptical mirror and the parabolic mirror may be disposed with a boundary adjacent to a plane which contains a straight line connecting the two focal points of the elliptical mirror and which is parallel to the lengthwise direction of the two orthogonal directions. The plane mirror may be disposed perpendicularly to parallel light as reflected by the parabolic mirror.
In one preferred form of the present invention, the auxiliary reflection mirror may comprise two parabolic surface mirrors having its focal point placed adjacent to the center of the light source, as well as a hyperboloids mirror having two focal points placed substantially at the same positions as the two focal points of the elliptical mirror. The elliptical mirror and the parabolic mirror may be disposed with a boundary adjacent to a plane which contains a straight line connecting the two focal points of the elliptical mirror and which is parallel to the lengthwise direction of the two orthogonal directions. The hyperboloids mirror may be disposed so that, of the light directed by the elliptical mirror to the optical integrator, those light rays having an angle, with respect to the optical axis, not smaller than a predetermined angle, are reflected thereby.
In one preferred form of the present invention, the auxiliary reflection mirror may comprise two parabolic surface mirrors having its focal point placed adjacent to the center of the light source, as well as a spherical surface mirror having its center placed adjacent to a focal point of the elliptical mirror, defined at a side remote from the light source. The elliptical mirror and the parabolic mirror may be disposed with a boundary adjacent to a plane which contains a straight line connecting the two focal points of the elliptical mirror and which is parallel to the lengthwise direction of the two orthogonal directions. The spherical mirror may be disposed so that, of the light directed by the elliptical mirror to the optical integrator, those light rays having an angle, with respect to the optical axis, not smaller than a predetermined angle, are reflected thereby.
In one preferred form of the present invention, the auxiliary reflection mirror may comprise two parabolic surface mirrors having its focal point placed adjacent to the center of the light source, as well as a spherical surface mirror having its center placed adjacent to the center of the light source. The elliptical mirror and the parabolic mirror may be disposed with a boundary adjacent to a plane which contains a straight line connecting the two focal points of the elliptical mirror and which is parallel to the lengthwise direction of the two orthogonal directions. The elliptical mirror may be disposed so that, of the light directed by the elliptical mirror to the optical integrator, those light rays having an angle, with respect to the optical axis, not smaller than a predetermined angle, are reflected thereby. The spherical mirror may be disposed so that it reflects any stray light from the elliptical mirror and also so that it does not intercept the light directed by the elliptical mirror to the optical integrator.
In summary, it is effective that the light collecting means is provided with an opening having different lengths along the two orthogonal directions; that the light collecting means is formed with a reflection surface having a shape being different in the two orthogonal directions; that the light collecting means is provided by two elliptical mirrors having the same focal point positions and having different lengths in major and minor diameters; or that the light collecting means comprises a main reflection mirror for directing light to the optical integrator and an auxiliary reflection mirror for directing, to the main reflection mirror, those light rays from the light source which do not directly impinge on the main reflection mirror. Also, it is effective that the main reflection mirror has an opening of substantially rectangular shape.
Further, in one preferred form of the present invention, the main reflection mirror may comprise an elliptical mirror having a first focal point placed about the center of the light source, while the auxiliary reflection mirror may comprise a spherical mirror having its center placed adjacent to the center of the light source. The elliptical mirror has an opening of a substantially rectangular shape. The spherical mirror may be disposed so that it reflects any stray light from the elliptical mirror and also so that it does not intercept the light directed by the elliptical mirror to the optical integrator. Alternatively, the main reflection mirror may comprise an elliptical mirror having a first focal point placed about the center of the light source, while the auxiliary reflection mirror may an elliptical mirror having a first focal point placed about the center of the light source, while the auxiliary reflection mirror may comprise a hyperboloids surface mirror having two focal points placed substantially at the same positions as the two focal points of the elliptical mirror. The hyperboloids mirror may have an opening of substantially rectangular shape.
In one preferred form of the present invention, the main reflection mirror may comprise an elliptical mirror having a first focal point placed about the center of the light source, while the auxiliary reflection mirror may comprise a plane mirror disposed between the two focal points of the elliptical mirror and being orthogonal to a straight line connecting the two focal points. The plane mirror may have an opening of substantially rectangular shape. Alternatively, the main reflection mirror may comprise an elliptical mirror having a first focal point placed about the center of the light source, as well as a hyperboloids mirror having two focal points placed substantially at the same positions as the two focal points of the elliptical mirror, while the auxiliary reflection mirror may comprise a hyperboloids surface mirror disposed between the two focal points of the elliptical mirror and being orthogonal to a straight line connecting the two focal points of the elliptical mirror. The hyperboloids mirror as the auxiliary reflection mirror may have an opening of substantially rectangular shape. The hyperboloids mirror as a portion of the main reflection mirror may be disposed so that is reflects light as reflected by the hyperboloids mirror of the auxiliary reflection mirror to direct the same tot he optical integrator.
Further, in one preferred form of the present invention, the main reflection mirror may comprise an elliptical mirror having a first focal point placed about the center of the light source, as well as a hyperboloids mirror having two focal points placed substantially at the same positions as the two focal points of the elliptical mirror, while the auxiliary reflection mirror may comprise a plane mirror disposed between the two foal points of the elliptical mirror and being orthogonal to a straight line connecting the two focal points. The plane mirror may have an opening of substantially rectangular shape. The hyperboloids mirror as a portion of the main reflection mirror may be disposed so that it reflects light as reflected by the plane mirror to direct the same to the optical integrator.